Simultaneous R2* and quantitative susceptibility mapping measurement enables differentiation of thrombus hematocrit and age: an in vitro study at 3 T

Background The efficacy of acute ischemic stroke treatment is affected by thrombus composition and age, yet no diagnostic method capable of quantitative thrombus characterization currently exists. This in vitro study evaluates the use of R-2*, quantitative susceptibility mapping (QSM), and proton density fat fraction (FF) maps derived from a single gradient echo (GRE) MRI acquisition for characterizing clot of various hematocrit, as well as added calcified and lipidic components, throughout aging. Methods Two thrombus phantoms containing porcine clots (10-60%hematocrit, one with added calcium or lard) were scanned serially throughout 6 days of aging. Three-dimensional multi-echo GRE imaging was used to generate R-2*, QSM, and FF maps, from which mean values for all clots at every time point were obtained. Receiver operating characteristic analysis was used to derive thresholds differentiating acute from chronic clot, and measured R-2* and QSM were tested for their ability to estimate clot hematocrit. Results R-2* and QSM varied minimally over the first 6hours of aging (acute), and QSM was found to linearly relate to clot hematocrit. Beyond 6hours (chronic), R-2* and QSM increased considerably over time and hematocrit could be estimated from the R-2*/QSM ratio. R-2* and QSM thresholds of 22s(-1) and 0.165 ppm differentiated acute from chronic clots with a sensitivity/specificity of 100%/100% and 85%/92%, respectively. QSM and FF maps definitively distinguished calcium and lipid, respectively, from clots of any hematocrit and age. Conclusions R-2*, QSM, and FF from a single multi-echo GRE scan discriminated hematocrit and age, and distinguished calcification and lipid withinin vitro clot.